U.S. patent number 4,554,802 [Application Number 06/589,247] was granted by the patent office on 1985-11-26 for adjustment of stitch cams in a knitting machine.
This patent grant is currently assigned to H. Stoll GmbH & Company. Invention is credited to Ernst Goller, Jurgen Ploppa, Fritz Walker.
United States Patent |
4,554,802 |
Goller , et al. |
November 26, 1985 |
Adjustment of stitch cams in a knitting machine
Abstract
A carriage 14 is reciprocable along a needle bed of a flat
knitting machine and carries a needle cam. A pair of stitch cams 12
and 13 are adjustably supported by the carriage 14 and are disposed
so as to trail behind the needle cam in alternate strokes of the
carriage, respectively. An adjustment mechanism 11 includes a
stepping motor 38 (FIG. 3) which adjusts the trailing stitch cam 12
or 13 not only at the beginning of the respective stroke but also
during the course of the stroke itself, thereby enabling the stitch
density to be varied within each row of the knitted article as well
as from row to row.
Inventors: |
Goller; Ernst (Reutlingen,
DE), Ploppa; Jurgen (Pfullingen, DE),
Walker; Fritz (Kusterdingen, DE) |
Assignee: |
H. Stoll GmbH & Company
(DE)
|
Family
ID: |
6194487 |
Appl.
No.: |
06/589,247 |
Filed: |
March 13, 1984 |
Foreign Application Priority Data
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Mar 24, 1983 [DE] |
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3310671 |
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Current U.S.
Class: |
66/71; 66/75.2;
66/77; 66/78 |
Current CPC
Class: |
D04B
15/327 (20130101) |
Current International
Class: |
D04B
15/32 (20060101); D04B 15/00 (20060101); D04B
007/00 (); D04B 015/36 () |
Field of
Search: |
;66/71,77,78,75.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2622883 |
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Dec 1975 |
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DE |
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2622347 |
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Dec 1976 |
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DE |
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Primary Examiner: Feldbaum; Ronald
Attorney, Agent or Firm: Jones, Tullar & Cooper
Claims
We claim:
1. In a knitting machine having a carriage movable along a needle
bed through successive strokes, a pair of stitch cams adjustably
supported by said carriage to trail behind a needle cam, mounted on
said carriage, during alternate ones of said carriage strokes,
respectively, and an adjusting mechanism including a stepping motor
operative to adjust each of said stitch cams relative to said
carriage at a beginning of the respective carriage stroke, the
improvement comprising said adjusting mechanism including said
stepping motor also being operative during the stroke of said
carriage to adjust the trailing one of said stitch cams.
2. In a knitting machine having a carriage movable along a needle
bed through successive strokes, a pair of stitch cams adjustably
supported by the carriage to trail behind a needle cam, mounted on
the carriage, during alternate ones of the carriage strokes,
respectively, and an adjusting mechanism including a stepping motor
operative to adjust each of the stitch cams relative to the
carriage at a beginning of the respective carriage stroke, the
improvement comprising a guide arrangement to guide the stitch cams
during adjustment relative to the carriage, wherein:
said adjusting mechanism including said stepping motor also being
operative to adjust the trailing one of the stitch cams during each
said carriage stroke; and
said guide arrangement includes for each of the stitch cams an
inclined guide slot in said carriage, a guide peg mounted on the
stitch cam and slidably engaged in the guide slot, an adjusting
ledge driven by the stepping motor for movement at an angle to the
guide slot, biasing means urging the guide peg into engagement with
the adjusting ledge, a lever on which the guide peg is mounted, and
an axle supporting the lever for rotation relative to the carriage,
said axle being disposed parallel to said guide peg.
3. The adjusting mechanism according to claim 2, wherein said guide
peg has a side facing away from said adjusting ledge, and said
lever is disposed on said side of said guide peg.
4. The adjusting mechanism according to claim 3, wherein said
adjusting ledge is movable vertically by said stepping motor.
5. The adjusting mechanism according to claim 2, wherein a pair of
said adjusting ledges are provided and are movable in mutually
parallel directions by said stepping motor, each of said adjusting
ledges being associated with a respective one of said stitch cams,
and further comprising a movable driving element connected to said
stepping motor, said driving element being disposed between said
adjusting ledges to act thereon.
6. The adjusting mechanism according to claim 5, wherein said
stepping motor has a rotatable drive spindle which includes an
externally threaded portion, and an internally threaded bushing is
engaged with said threaded portion for axial movement along said
drive spindle upon rotation thereof, said bushing being positively
connected without great play to said driving element.
7. The adjusting mechanism according to claim 6, wherein said
driving element comprises a rotatable eccentric member and a slide
part which is moved towards and away from an outer periphery of
said bushing by rotation of said eccentric motor.
8. The adjusting mechanism according to claim 7, wherein said
eccentric member is composed of a shank and a pair of journals
provided at ends of said shank respectively, said shank being
disposed eccentrically with respect to said journals and engaging
said slide part.
9. The adjusting mechanism according to claim 5, further comprising
an intermediate member held in interchangeable fashion between said
driving element and said ajusting ledges.
10. The adjusting mechanism according to claim 5, further
comprising a correction member disposed between said driving
element and said adjusting ledges, said correction member being
firmly connected to said driving element.
11. The adjusting mechanism according to claim 5, further
comprising a stop device to define a lowermost position of said
driving element.
12. The adjusting mechanism according to claim 6, further
comprising a stop device to define a lowermost position of said
bushing, said stop device being composed of a projection extending
axially from said bushing and a pin projecting radially from said
drive spindle, said lowermost position being defined by said
projection lying in a path of revolution of said pin.
13. The adjusting mechanism according to claim 12, wherein said
externally threaded portion of said drive spindle has screw threads
of a predetermined pitch, and said projection and said pin are
separated axially of said drive spindle by a distance corresponding
to an integral multiple of said predetermined pitch each time said
pin becomes axially aligned with said projection.
14. The adjusting mechanism according to claim 5, further
comprising limiting means to define an uppermost limit on movement
of said driving element.
15. The adjusting mechanism according to claim 6, further
comprising limiting means to define an uppermost limit on movement
of said bushing, said limiting means including a pressure spring
which is compressed when said bushing becomes unthreaded from said
threaded portion of said drive spindle.
16. The adjusting mechanism according to claim 7, further
comprising limiting means to define an uppermost limit on movement
of said bushing, said limiting means including a pressure spring
which encircles said eccentric member, said pressure spring being
compressed when said bushing becomes unthreaded from said threaded
portion of said drive spindle.
17. The adjusting mechanism according to claim 2, wherein said
stepping motor is a multiphase motor.
18. The adjusting mechanism according to claim 17, wherein said
multi-phase motor has a multiplicity of phases including a selected
phase, said motor being operable through successive steps by
energisation of said phases, and further comprising a control to
arrest operation of said motor at a zero position of each said
stitch cam, said control including a microswitch which is operated
when said motor is a few steps away from said zero position and
means to de-energise said motor in response to operation of said
microswitch when said selected phase is energised.
19. The adjusting mechanism according to claim 2, wherein said
stepping motor has a drive shaft, and further comprising a spindle
firmly secured to said drive shaft, a radial bearing supporting
said spindle, and a pressure spring acting on said radial bearing
to prestress said radial bearing.
Description
This invention relates to the adjustment of stitch cams of a
knitting machine, particularly a flat knitting machine.
Typically, a flat knitting machine comprises a needle bed along
which a carriage moves through forward and return strokes, the
carriage having a needle cam and a pair of stitch cams mounted
thereon. An adjustment mechanism is provided for adjusting the
stitch cams relative to the carriage immediately before the return
stroke of the latter, such adjustment usually being performed by
means of a stepping motor.
On most flat knitting machines of this kind which are in use at
present, the adjusting mechanism is of purely mechanical design and
includes various slides which are assigned to the stitch cams.
These slides are actuated selectively at the stroke return point of
the carriage, so that the setting of the stitch cam trailing behind
the needle cam can be selected for operation. These purely
mechanical adjusting mechanisms are unsatisfactory because, for
reasons of design, they only provide a few setting possibilities
for the stitch cams.
Recently, therefore, a change has been made to the use of adjusting
mechanisms which utilise a stepping motor for setting the stitch
cams at an essentially infinitely variable number of positions.
This gives rise to a considerable increase in the number of
different knitting styles which can be obtained owing to the number
of different stitch densities which can be achieved. Examples of
adjusting mechanisms of this type are disclosed in German OS No. 21
11 553 and German OS No. 21 53 429.
In the adjusting mechanism described in the last-named document, a
stepping motor is rigidly connected to each stitch cam, which means
that the stitch cam is switched off with the aid of the respective
stepping motor and through this each stitch cam always has to be
re-set. Switching off of the one stitch cam and switching on of the
other stitch cam by adjusting the associated stepping motor occurs
at the point of carriage return, i.e. immediately before the return
stroke before the actual knitting operation, as with the
previously-described mechanical adjusting mechanisms. A time
problem therefore arises, along with a problem regarding the
reproducibility of the setting of the stitch cams.
In the adjusting mechanism described in the first-named document, a
lift-off device is provided by means of which the stitch cam not in
operation, along with its associated adjusting mechanism including
the stepping motor, is lifted from the needle bed. With this
construction, the time problem mentioned above is avoided because
the setting and adjustment of the stitch cam can be done during the
carriage stroke in the raised, non-working position. As before,
however, at the return point of the carriage stroke, that is before
the actual knitting operation, the adjusting mechanism (together
with the stitch cam which has been brought into its pre-selected
position) is brought towards the needles, i.e. into operation. In
both of the above constructions, a separate stepping motor is
assigned to each stitch cam and is disposed such that its adjusting
movement acts in the direction of displacement of the stitch
cam.
In German OS No. 26 22 883 there is disclosed an adjusting
mechanism in which the two stitch cams, which are directed
slopingly towards one another, can be actuated by a horizontally
movable cam slide rod, the rod having a toothed rack end which can
be driven by a stepping motor. Using such a cam rod, it is possible
not only to bring the stitch cams into and out of action, but also
to achieve height adjustment of the stitch cams within certain
limits. Movement of the cam slide rod occurs at the point of return
of the carriage stroke. Since the cam rod not only exerts a
movement in an adjusting direction on these stitch cams but also
produces a movement component perpendicularly to this, and since
the height adjustment of the stitch cams is done by the cam rod
without their being lifted up, a very strong stepping motor is
needed which is disadvantageous both from the point of view of cost
and also in respect of the weight which must be moved by the
carriage. A further disadvantage of this adjusting mechanism
results from the fact that, at each carriage return point, height
adjustment for the stitch cams has to be re-executed because the
previous height adjustment is lost when one or other of the stitch
cams is brought into or out of action. This results not only in a
setting which is difficult to reproduce but also in a considerable
amount of time being needed for the setting.
In all of the above-mentioned conventional adjusting mechanisms,
the stitch cam is preselected in its non-working position and is
only brought into the operating position at the carriage return
point, there being no further adjustment during the carriage
stroke. This means that the stitch density cannot be altered within
the rows themselves and each row must therefore have a uniform
stitch density.
It is an object of the present invention to avoid this negative
effect and to provide an adjusting mechanism in which it is
possible to vary the stitch density not only in the longitudinal
direction but also transversely of the piece of knitting.
According to the present invention, there is provided a knitting
machine comprising a needle bed, a carriage reciprocable along the
needle bed and having a needle cam mounted thereon, a pair of
stitch cams adjustably supported on the carriage and being disposed
so as to trail behind the needle cam during alternate strokes of
the carriage, respectively, and an adjusting mechanism including a
stepping motor operative to adjust the trailing stitch cam relative
to the carriage not only at the beginning of the respective stroke
but also during the actual stroke itself.
Since the relevant stitch cam can be adjusted during the actual
stroke of the carriage as well as at the return point thereof, it
is possible to vary the stitch density of the piece of knitting in
a transverse direction as well as in the direction of knitting.
This makes it possible to produce pieces of knitting in variety
because the setting of the stitch cams can be varied for each
individual stitch within each row.
Although it is known from Swiss Patent Specification No. 465 117 to
adjust the stitch cams additionally during the carriage stroke with
the aid of a mechanical adjusting mechanism, this is done by means
of relatively complicated and expensive guide bars provided with
adjustable cam carriers on which a lever device acts, the lever
device being connected to sliding elements of the stitch cams.
Nevertheless, even using this known type of mechanical adjusting
mechanism, it is not possible to provide varying combinations of
knitting density in the individual rows because the carriers give
the same stitch density in each row once they have been set. A
certain increase in the variety of the pieces of knitting can
indeed be achieved, but only to a relatively limited extent. For
this reason, this type of mechanical adjusting mechanism has not
been taken up very widely in practice.
In a preferred example of the adjusting mechanism, each of the
stitch cams is guided by means of a guide peg which is slidably
engaged in a guide slot, this slot being inclined to the axis of
the carriage. The guide pegs abut against an adjusting ledge under
the action of a tension spring, the adjusting ledge being driven by
the stepping motor in a direction inclined to the direction of the
guide carriage. The guide pegs are connected to a guide lever in
such a manner that they can swivel, the other end of the guide
lever being held so that it can swivel on an axle disposed parallel
to the guide peg and fixed to the lever. In this way, even when
both of the stitch cams are driven by a common stepping motor, it
is possible to carry out a height adjustment in a condition wherein
the motor is loaded with at least one of the stitch cams. The guide
lever takes up the motive force component transversely of the guide
slot, so that tilting and friction which considerably affect the
design of the stepping motor are avoided. It is thus no problem to
adjust the stitch cam in question quickly and accurately even
during the stroke of the carriage and to knit simultaneously with
this new setting.
The guide lever is desirably disposed on a side of the guide peg
which faces away from the adjusting ledge, whereby the adjusting
ledge can be moved vertically.
A simple design is made available if two adjusting ledges are
provided which can be moved parallel to one another, these ledges
being acted upon by a common driving element disposed there-between
and connected to the stepping motor. With such an arrangement, with
the height setting remaining the same during forward and return
strokes, the adjustment of both adjusting ledges can take place by
means of a single adjustment operation.
In order to achieve height adjustment of the stitch cams which is
as accurate as possible, which is predetermined and which is
reproducible, the driving element can be positively connected
without great play to an internally threaded bushing, the bushing
being engaged with a spindle shaft of the stepping motor for
movement therealong. This can be achieved by providing the driving
element with a slide part which is movable towards the outer
periphery of the bushing, the slide part being thus movable by
means of an eccentric member extending perpendicularly to the
direction of sliding. In this manner, by rotating the eccentric
member to slide part is pressed against the bushing, creating a
friction lock which prevents any play which might otherwise
influence the adjusting precision of the driving element.
In order to adapt the adjusting mechanism to differing sizes of
needle, an intermediate member may be held in interchangeable
fashion between the driving element and the adjusting ledges. By
changing this intermediate member, a parallel displacement of the
zero setting of the adjusting mechanism is produced which does not
however necessitate the same sort of change in the motor and switch
setting. In this way, the average adjustment time required for the
stitch cams remains unextended, and yet an adjustment or adaptation
to the fineness of the knitting machine is possible.
In order to be able to balance out the mechanical tolerances
inherent in the machine, a correction member firmly connected to
the driving element can be disposed between the driving element and
the adjusting ledges. This connection member is selected as a
function of the mechanical tolerance on the one hand of the
adjusting mechanism and on the other hand of the fixing on the
carriage in relation to the stitch cams.
To fix the zero point in a lower-most position of the driving
element, a stop device can be provided which comprises two stop
elements movable relative to one another, these elements being able
to come up against each other in a peripheral direction. This
prevents the bushing from hitting against an axial stop, which
would have the effect of leading to automatic locking which could
not be released again by the stepping motor without considerable
difficulty. The setting here can be such that a peg projecting
radially from the spindle goes just past a pin projecting axially
from the bushing in the penultimate revolution, while in the next
revolution it hits against this pin because the projection arrives
in the peripheral path of the latter.
A limitation can also be provided on the upward travel of the
bushing in a similar way to the lower travel limitation or zero
point setting mentioned above. For example, the bushing can be
moved against a pressure spring which compresses when the bushing
becomes unthreaded from the spindle shaft. In other words, as soon
as the bushing compresses the pressure spring, it disengages from
the external thread on the spindle shaft so that no further axial
adjustment occurs. Nevertheless, if the stepping motor is turned
back, the two threads can interlock again because the pressure
spring presses the bushing against the threaded part of the spindle
shaft.
The invention will now be further described, by way of example
only, with reference to the accompanying drawings, in which:
FIG. 1 is a plan view of a carriage of a knitting machine according
to the present invention, the carriage supporting two stitch cams
and an adjusting mechanism therefor;
FIG. 2 is a section taken along the line II--II in FIG. 1;
FIG. 3 is a longitudinal section through the adjusting mechanism,
the section being taken in a plane at right angles to that of FIG.
1;
FIG. 4 is a view along arrow IV in FIG. 3; and
FIG. 5 is a section taken along the line V--V in FIG. 3.
FIG. 1 shows a carriage 14 which is reciprocable along a needle bed
(not shown) of a flat knitting machine, the carriage 14 supporting
a needle cam (also not shown) and a pair of stitch cams 12 and 13
which can be adjusted by means of an adjusting mechanism 11. As
will be explained in detail later on, the adjusting mechanism 11 is
constructed so that whichever one of the stitch cams 12 and 13
trails behind the needle cam in a given stroke of the carriage, it
can be adjusted in height during the actual carriage stroke. This
enables the closeness of the individual stitches to be selected
differently not only from row to row in the knitted article, but
also from stitch to stitch in each row, and at the same time the
knitting can be made correspondingly more or less close.
Of the carriage 14, FIG. 1 shows essentially only an upper or front
plate 16 which faces away from the needle bed, on which plate are
fixed the individual elements of the adjusting mechanism 11, the
stitch cams 12 and 13 being disposed in a plane behind this (see
FIG. 2). Other than this, the carriage 14 and the plate 16 are
shown only to the extent necessary to illustrate the stitch cams 12
and 13 with the adjusting mechanism 11.
The two stitch cams 12 and 13 are disposed in a conventional way
symmetrically relative to an imaginary longitudinal center plane 17
of the needle cam, in such a way that they are inclined towards one
another. The stitch cams 12 and 13 are held on a rear side of a
lower or rear plate 18, and are movable along respective inclined
guideways 19 in the direction indicated by double-headed arrows A
and A'. As can be seen in FIG. 2, each stitch cam is attached to an
approximately L-shaped carrying member 21 which is also able to
move in the direction of the arrow A or A'. The carrier member 21
lies with a long arm 22 thereof on the front end of the back plate
18, i.e. opposite the needle lowering device which is connected
through the back plate 18 to the carrier member 21. The stitch cam
or the carrier member 21 associated therewith is connected to one
end of a tension spring 24, the other end of which is secured to a
fixed part, for instance the back plate 18. The spring 24 extends
in the direction of the arrow A or A', and thus endeavours to pull
the stitch cam 12 or 13 into a lowermost position.
Under the action of the tension spring 24, a short arm 23 of the
carrier member 21 abuts against a radial ball bearing 26 which is
fixed on the back end of a guide peg 27. The peg 27 extends in a
direction perpendicular to the stitch cam 12 or 13 between the
plates 16 and 18. In an end area facing towards the front plate 16,
the guide peg 27 is pivotably mounted on one end of a guide lever
28, whereby the guide lever 28 is penetrated by the guide peg 27
and abuts against a rear flange 29 of the peg 27. The other end of
the guide lever 28 has an elongated bearing bushing 31 which is
pivotably mounted on a fixed axle 32 extending parallel to the
guide peg 27.
The front end of each guide peg 27 abuts via a radial ball bearing
33 against one of two adjusting ledges 36 and 37 which, with the
aid of a common stepping motor 38 (see FIG. 3), can be reciprocated
in a vertical direction as indicated by double-headed arrow B. The
adjusting ledges 36 and 37 are attached respectively to a pair of
slides 41 and 42 which extend parallel to each other and which are
each guided for movement along a slot 43 in a respective guide
plate 44, 45 of a driving unit 39 which contains the stepping motor
38. Each slide 41, 42 is fitted with a roller 46, 47 on an end
thereof which faces away from the adjusting ledge 36, 37. This
roller co-operates with an adjusting slide 48 which extends
perpendicularly thereto and which is provided with a cam plate 49.
The adjusting slide 48, which can be moved backwards and forwards
in the direction of the double-headed arrow C, determines whether
and which of the stitch cams 12 and 13 comes into action. In FIG.
1, the left-hand stitch cam 12 is depicted as being out of action
because it is pushed by the guide peg 27 and the adjusting ledge 36
into an uppermost, non-working position: in contrast, the
right-hand stitch cam 13 is shown as being ready to operate. The
adjusting slide 48 can however also be positioned such that both of
the stitch cams 12 and 13 are out of action, which is the case when
the rollers 46, 47 of both slides 41, 42 are pushed upwards by the
adjusting slide 48. The return of the slides 41 and 42 when
released by the adjusting slide 48 occurs under the action of the
respective tension springs 24 acting on the carrier members 21.
The driving unit 39 will not be described with reference to FIGS. 3
to 5. The driving unit 39 is secured along with the guide plates 44
and 45 on the front plate 16 of the carriage 14, being fixed by
means of a fitting pin 51. The plate 44 is secured on a housing 52
on whose lower end the stepping motor 38 is flange-mounted (see
FIG. 3) a drive shaft 53 of the motor 38 being firmly attached to a
threaded spindle 56 by way of connecting sleeve 54 so that it
cannot rotate and is fixed in the axial direction. The threaded
spindle 56 penetrates through an axial recess 57 in the housing 52
and is rotatably mounted on an end of the housing 52 remote from
the motor 38 by means of a radial bearing 58 and a thrust bearing
59.
A threaded bushing 61 is mounted on the spindle 56 and is held
against rotation in the peripheral direction, so that it can be
moved to and fro in an axial direction (as indicated by the
double-headed arrow B) by rotation of the spindle 56. To prevent
rotation thereof, the bushing 61 is positively connected, e.g. by a
frictional locking as will be described later, to a connecting
element 62 which is held so that it cannot be rotated but can slide
a little in a transverse direction. Such positive connection is
achieved by engagement of the outer circumference of the bushing 61
in a recess 63 formed in the connecting element 62.
An eccentric member 67 is disposed parallel to the threaded spindle
56 and has a shank 66 which passes through a bore 68 in the
connecting element 62. At one end thereof the shank 66 has a
journal 69 while at the other end thereof it has a head 71 provided
with a screwdriver slot, the journal 69 and the head 71 being
disposed a few tenths of a millimeter eccentrically to the shank 66
and being mounted so that the member 67 can be rotated. By rotating
the eccentric member 67, the connecting element 62 is moved in the
direction of the outer circumference of the internally threaded
bushing 61 because of the eccentricity of the shank 66 relative to
the axis of rotation of the journal 69 and the head 71. This makes
it possible for the connecting element 62' to be pressed onto the
internally threaded bushing 61 thereby achieving the aforementioned
frictional locking to prevent the otherwise unavoidable play
between the spindle 56 and the bushing 61 from affecting the
precision of the adjustment of the mechanism 11. As can also be
seen from FIG. 3, the connecting element 62 has on its bottom end a
radially projecting driver pin 72 which indirectly is in or can be
brought into an operative connection with the two adjusting ledges
36 and 37.
Between the driver pin 72 and lower edges 73 of the two adjusting
ledges 36 and 37, a correction member or block 74 and an additional
member or block 76 are disposed. The correction block 74 is
provided in order to compensate for the manufacturing tolerances
which are inherent to the mechanism. On the flat knitting machine
which is to be fitted with the adjusting mechanism 11, the vertical
distance between the driver pin 72 in zero position and the fitting
pin 51 on the carriage 14 (or the bore in the driving unit 39 into
which this pin 51 fits) is measured, and according to the deviation
from the desired value a correction block 74 of suitable size is
selected and fixed on the connecting element 62. In contrast, the
additional block 76 (which is held so that it can be interchanged)
serves to adapt the adjusting mechanism and more particularly the
driving unit 39 thereof to differing degrees of fineness of the
knitting or of the needles used in the knitting machine. Depending
on the selected fineness, a larger or smaller stroke range or a
transposing of the stroke range upwards or downwards is necessary
for the stitch cams 12 and 13. This is achieved by using a longer
or a shorter additional block 76, which results only in a parallel
displacement of the zero position to be set directly on the driving
unit 39. This is achieved by using a longer or a shorter additional
block 76, which results only in a parallel displacement of the zero
position to be set directly on the driving unit 39. This parallel
displacement can however be considered without altering the zero
position.
Movement of the internally threaded bushing 61 and hence the
connecting element 62 is limited both upwardly and downwardly. The
lowermost limit is defined by a stop pin 81 which projects radially
from the motor shaft in the region of the connecting sleeve 54, and
also by a stop projection 82 which projects from the internally
threaded bush 61 downwardly in an axial direction. When the bushing
61 approaches the region of its lowermost movement limit, the
projection 82 becomes disposed in the peripheral path of movement
of the pin 81 so that the latter then comes up against the
projection 82 in a peripheral direction. The spatial positioning of
the pin 81 and the projection 82 is such that, at the end of the
penultimate revolution of the shaft 53 prior to the bushing 61
reaching its lowermost position, the pin 81 can just still move
beneath the projection 82.
The upper limit of movement of the bushing 61 and the adjusting
element 62 is determined by the internally threaded bushing 61
becoming disengaged from the external threading on the spindle 56
when the maximum stroke is reached. This means that the external
threading terminates at a certain distance from the end of the
spindle 56 which is held in the bearings 58 and 59. In order to
ensure a simple rethreading of the bushing 61 onto the threading of
the spindle 56 when the stepping motor 38 is turned backwards, a
pressure spring 83 is provided in the path of the connecting
element 62 so that it becomes engaged by the element 62 as the
bushing 61 approaches its maximum stroke. In the illustrated
construction, the spring 83 surrounds the shank 66 of the eccentric
member 67 with one end thereof supported on the housing 52: as an
alternative, however, the spring 83 could surround the threaded
spindle 56. The pressure spring 83 is of such a length that it is
compressed to a certain degree even before the bushing 61 becomes
unthreaded from the spindle 56, so that the counter-pressure
produced by the spring 83 after the bushing 61 has become
unthreaded, exerts an axial force on the bushing to encourage
rethreading to take place.
In FIG. 5, the zero point setting of the driving unit 39 is shown.
The stepping motor 38 is a four-phase stepping motor which executes
24 steps per complete revolution of 360.degree., i.e. the drive
shaft thereof moves 15.degree. per step. The individual phases of
the motor 38 are driven alternately with a pulse so that the
individual phases in alternation move the rotor by 15.degree. per
pulse. In the region of the lowermost limit of movement of the
bushing 61 a microswitch 84 is provided: this microswitch is
operated when an angle of less than 60.degree. (e.g. 30.degree. or
two steps) remains between the stop pin 81 and the projection
82.
At the same time, one of the phases of the stepping motor is chosen
to define the end or zero position. The zero point setting is
performed such that, when moving towards the zero point position,
each time the chosen phase of the stepping motor 38 is in
operation, a determination is made whether or not the microswitch
84 is closed. If the microswitch is not closed, then the motor is
moved further. If, on the other hand, the microswitch is closed
then the motor 38 is shut off because the zero point position has
been reached. The switching time of the microswitch 84 can
therefore vary within the above-mentioned remaining angle of
rotation without altering the zero position. In this way, the zero
point of the driving unit 39 is reproducible at any time, even
after the bushing 61 has become unthreaded from the spindle 56.
Since each of the stitch cams 12 and 13 is springloaded, that is it
can move in an upward direction also during operation, vibrations
can occur which knock the spindle 56 and the shaft 53 of the
stepping motor 38. In order to protect the bearings and the rotor
of the stepping motor 38 from damage, on the one hand the radial
bearing 58 which supports the threaded spindle 56 is mounted in the
housing 52, and on the other hand a pressure spring is provided
which, inside the housing of the stepping motor, encircles the
drive shaft 53 and prestresses this so that inside the motor 38 the
armature cannot hit against the thrust bearing provided there.
* * * * *